X-ray equipment used in the medical field typically includes a rotating anode X-ray tube. Such X-ray tubes are vacuum tubes each including a rotor having a rotatable shaft and each also including a stator which circumferentially surrounds, or is circumferentially surrounded by, the rotatable rotor shaft. An anode target surface is attached to, and rotated by, the rotor shaft. Electrons from a heated cathode strike the anode target surface to produce X-rays.
A pair of bearings is positioned radially between the rotor shaft and the stator. In a straddle-bearing design, the heavy anode target is located longitudinally between the two bearings, and the bearing arrangement is supported at both axial ends. Typically, each bearing includes a radially-outer race having a circumferential ball-bearing raceway surface, a radially-inner race having a circumferential ball-bearing raceway surface, and a set of circumferentially-arrayed ball bearings positioned radially between, and in rolling contact with, the raceway surfaces of the radially-outer and radially-inner races.
Usually, each of the two bearings is a pre-assembled bearing unit which is inserted between the rotor shaft and the stator. Since there is a clearance between the radially-inner races and the stator and a clearance between the radially-outer races and the rotor shaft, it is difficult to maintain the concentricity of the races which can lead to uneven wear of the ball bearings resulting in increased noise and reduced life of the X-ray tube. Also, under high speed rotation, the radially-inner and radially-outer races can start moving with respect to the rotor shaft and the stator making the bearing even more noisy.
Another known design replaces the pre-assembled bearing units with two radially-inner raceway surfaces which are integral with the stator, two radially-outer races which are separate pieces from the stator and the rotor shaft and which slide inside the rotor shaft, and two sets of ball bearings. Since there is a clearance between the radially-outer races and the rotor shaft, it is difficult to maintain the concentricity of these races which can lead to uneven wear of the roller bearings resulting in increased noise and reduced life of the X-ray tube. Also, under high speed rotation, the radially-outer races can start moving with respect to the rotor shaft making the bearing even more noisy. It is noted that radially-paired raceway surfaces together define load-bearing axes for the two sets of ball bearings. Such load-bearing axes typically are non-orthogonally-aligned with respect to the longitudinal axis of the rotor shaft, and a spring is used to apply a fixed longitudinal preload to one of the two radially-outer races. The fixed pre-load applied against one of the two radially-outer races somewhat reduces the bearing noise.
What is needed is an X-ray tube assembly having bearings which better maintain the concentricity of the bearing races and which further reduce bearing noise.